Multicylinder thermodynamic reciprocating machine in which the fuel supply to burner devices is controlled by means of temperature-sensitive elements

ABSTRACT

A multicylinder thermodynamic reciprocating machine in which a fuel supply and a supply for air of combustion communicate with each burner device, in which the ends of the fuel supplies remote from the associated burner device communicate with a common fuel supply duct in which a pressure control device during operation maintains a constant pressure and the ends of the supplies for air of combustion remote from the associated burner device communicate with a common supply duct for air of combustion, in which, taken in the direction of flow, a flow restricting element and a fuel control mechanism are incorporated in each fuel supply, in which a selecting device for the automatic selection of the maximum or minimum fuel pressure from several fuel inlet pressures is present and the outlet sides of the flow restricting elements communicate with the same number of inlets of the selecting device, the pressure differential between the constant fuel pressure in the common fuel supply duct and the selected maximum or minimum fuel pressure influencing a control member which actuates a control mechanism for air of combustion in the common supply duct for air of combustion.

United States Patent 1191 Brandenburg et a1.

[ MULTICYLINDER THERMODYNAMIC RECIPROCATING MACHINE IN WHICH THE FUELSUPPLY TO BURNER DEVICES IS CONTROLLED BY MEANS OF TEMPERATURE-SENSITIVEELEMENTS [75] Inventors: Klaus Brandenburg, 1

Kirchen-Wehbach; Joachim Kuhlmorgen, Aachen, both of Germany [73]Assignee: U.S. Philips Corporation, New

York, NY.

[22] Filed: June 29, 1973 [21] App]. No.: 374,904 I Related US.Application Data [63] Continuation of Ser. No. 234,074, March 13,1972.

[30] Foreign Application Priority Data Mar. I8, 1971 Netherlands 713610[52] US. Cl. [50/524,69/525 [51] Int. Cl. F03g 7/06 [58] Field of Search60/23, 24, 33, 59

[56] References Cited UNITED STATES PATENTS 1,618,594 2/1927 Koenig60/24 2,664,698 1/1954 Van de Poll et a1 60/24 3,600,886 8/1971 Jasperset a1. 60/24 3,699,770 10/1972 Bennethom 60/24 11] 3,822,550 [45]Ju1y9,197

FQREIGN PATENTS OR APPLICATIONS 895,869 5/1962 Great Britain 150/24Primary Examiner-Wendell E. Burns Attorney, Agent, or Firm-Frank: R.Trifari [57] ABSCT A multicylinder thermodynamic reciprocating machinein which a fuel supply and a supply for air of combustion communicatewith each burner device, in which the ends of the fuel supplies remotefrom the associated burner device communicate with a common fuel supplyduct in which a pressure control device during operation maintains aconstant pressure and the ends of the supplies for air of combustionremote from the associated burner device communicate with a commonsupply duct for air of combustion, in which, taken in the direction offlow, a flow restricting element and a fuel control mechanism areincorporated in each fuel supply, in which a selecting device for theautomatic selection of the maximum or minimum fuel pressure from severalfuel inlet pressures is present and the outlet sides of the flowrestricting elements communicate with the same number of inlets of theselecting device, the pressure differential between the constant fuelpressure in the common fuel supply duct and the selected maximum orminimum fuel pressure influencing a control member which actuates acontrol mechanism for air of combustion in the common supply duct forair of combustion.

11 Claims, 9 Drawing Figures PATENTED- 91914 3.822.550

SHEE! Q [If 6 MULTICYLINDER THERMODYNAMIC RECIPROCATING MACHINE IN WHICHTHE FUEL SUPPLY TO BURNER DEVICES llS CONTROLLED BY MEANS OFTEMPERATURESENSITIVE ELEMENTS This is a continuation of application Ser.No. 234,074, filed Mar. 13, 1972.

The invention relates to a multicylinder thermodynamic reciprocatingmachine. Each cylinder comprises a heater to which thermal energy froman associated burner device can be supplied; a fuel supply and a supplyfor air of combustion communicate with each burner device, and the endsof the fuel supplies remote from associated burner device communicatewith a common fuel supply duct in which a pressure control devicemaintains a constant fuel pressure during operation. The ends of thesupplies for air of combustion remote from the associated burner devicecommunicate with a common supply duct for air of combustion. Each heatercomprises a temperature-sensitive element which actuates a fuel controlmechanism in the fuel supply of the associated burner device forcontrolling the fuel flow to the said device. Each fuel supply, taken inthe direction of flow, comprises before the fuel control mechanism. Aflow-restricting element across which during operation, a pressuredifferential prevails which is proportional to the rate of the flow offuel through the relevant supply. The quantity of air of combustion tobe supplied is controlled in accordance with the supplied quantity offuel.

A thermodynamic reciprocating machine of the type of the presentinvention, is known from British Pat. specification No. 895,869, whichdescribes for a monocylinder thermodynamic reciprocating machine, acontrol of the quantity of air of combustion to be supplied to theburner device in accordance with the quantity of fuel to be supplied. Inthis case, pressure difference gauges are arranged in the supplies forfuel and air of combustion which gauges actuate, independently of eachother in opposite senses, the same member of a hydraulic system whichcontrols the position of a throttle valve in the supply for air ofcombustion in accordance with the fuel flow in the fuel supply. Thisknown construction for controlling the supplies for fuel and air ofcombustion to the burner device of a monocylinder thermodynamicreciprocating machine exhibits a few drawbacks which make it lessattractive for use also in multicylinder thermodynamic reciprocatingmachines. First'of all, very high requirements must be imposed upon thecontrol mechanism in the fuel duct. As a matter of fact, the passageshould be adjustable accurately and in a reproduceable manner over alarge fuel flow range. When these requirements are not satisfied, thiswill give rise to all kinds of difficulties, notably in the case ofsmall fuel flows, such as extinguishing of the burner, instability ofthe temperature control circuit, incomplete combustion of the air-fuelmixture with dirty exhaust gases which are detrimental to health, allthis therefore as a result of the wrong dosing of fuel. For the pressuredifference gauge in the fuel supply duct, a great ratio for the largestto the smallest fuel flow means that said gauge must be capable ofmeasuring pressure differences accurately and in a reproduceable mannerwithin a very large measuring range. Actually, according to Bernouillistheorem, the produced pressure differential is proportional to thesquare of the rate of flow.

The use in multicylinder thermodynamic reciprocating machines of such asolution is furthermore even less attractive, since in that case thenumber of control systems required is the same as the number ofcylinders present. The air-fuel control in its totality then consists ofa very complicated assembly built up from a very large number ofcomparatively expensive components.

It is the object of the present invention to provide an air-fuel controlsystem for multicylinder thermodynamic machines which combinessimplicity in construction, including a comparatively small number ofcheap components, with a reliable operation.

In order to realize this objective, the multicylinder thermodynamicreciprocating machine according to the invention is characterized inthat it includes a selecting device for the automatic selection of themaximum or minimum fuel pressure from several fuel inlet pressures. Theoutlet sides of the flow restricting elements communicate with an equalnumber of inlets of the selecting device, the pressure differentialbetween the constant fuel pressure in the common fuel supply duct andthe selected maximum or minimum fuel pressure influencing a controlmember which actuates a control mechanism for air of combustion in thecommon supply duct for air of combustion.

By using the selecting device, which may have a very compact and simpleconstruction, a pressure difference gauge and throttle valve need not bepresent in each supply for air of combustion. This is in deviation fromthe already mentioned known control system, and as a result of this, aconsiderable saving of components is obtained. An evenmore importantsaving is obtained in this case, in that it is not necessary for eachburner device to separately compare the flows of air and fuel to theburner device and to match them mutually via an associated controlsystem such as, for example, the hydraulic system described in the saidlBritish Pat. specification No. 895,869. In this case only one controlsystem is necessary with which the air flow through the common supplyduct for air of combustion is adapted to the maximum fuel flow from thevarious fuel flows which flow through the fuel supplies of the burnerdevices by choosing a selecting device which selects the minimum fuelpressure.

By the resulting great saving of comparatively expensive components withthe introduction of a simple selecting device, a multicylinderthermodynamic reciprocating machine having an attractive, simple andcheap air-fuel control is obtained. Irrespective of the number ofcylinders, only one fuel pressure difference sensor is necessary towhich the fuel pressure difference signal is supplied.

Since furthermore the flows of fuel to the various .bumer devices aredirectly compared mutually via the ln multicylinder thermodynamicmachines in which the fuel flow of liquid fuel may vary per burnerdevice from, for example, 0.02 gram/sec. to 1.2 gram/sec (ratio 1 60),this means a ratio for the minimum to maximum pressure difference,produced preferably with one or more plates, of l 3600. Since this ratioand hence also the required measuring range is very large, the measuringinstrument becomes complicated and expensive. It is also substantiallyimpracticable to realize for small fuel flows of a few hundredths ofgrams per second a turbulent and hence temperatureindependent flow inthe plates.

A sufficiently reliable air-fuel control in the case of small fuel flowsthen is not possible and involves the danger of too small a ratio-fuelin the case of said small fuel flows, as a result of which incompletecombustion and dirty exhaust gases occur.

In order to avoid this drawback, a favourable embodiment of themulticylinder thermodynamic reciprocating machine according to theinvention comprises an auxiliary duct for fuel which communicates withone end with the common fuel supply duct. In this auxiliary duct aflow-restricting element is also incorporated on the inlet side of whichthe constant fuel pressure prevails. The outlet side of this duct alsocommunicates with an inlet of the selecting device, which selects theminimum fuel pressure. The fuel auxiliary duct furthermore incorporates,taken in the direction of flow after the flow-restricting element, afixed restriction which is chosen to be so that below a given minimumfuel flow to the fuel supplies, the pressure prevailing on the outletside of the flow restricting element in the fuel auxiliary duct, whichpressure is likewise constant, is lower than the pressures which prevailin that case on the outlet sides of the flow restricting elements in thefuel supplies.

it is achieved in this manner that below a given minimum fuel flow,irrespective of the value which the fuel flow then assumes, a constantvalue is obtained for the fuel pressure differential influencing thecontrol member. The supplied quantity of air of combustion then is alsoconstant and sufficiently large for any fuel flow below the saidminimum. Therefore, complete combustion with small fuel flows is ensuredin all cases.

In certain circumstances it may occur that the air flow originating fromthe common supply duct for air of combustion distributes insufficientlyuniformly between the various supplies for air of combustion, forexample, as a result of differences in counter pressures acting upon thesupplies from the associated burner devices or as a result of differentvalues of flow restricting elements of the said supplies. The dangerthen exists that a quantity of air of combustion which is insufficientto ensure complete combustion of the supplied quantity of fuel issupplied to a burner device. This would result again in dirty exhaustgases which are detrimental to health. For that reason it may sometimesbe desirable to have more certainty about the quantities of air flowingthrough the supplies for air of combustion, notably about the smallestflow of air of combustion.

For that purpose, a favourable embodiment of a multicylinderthermodynamic reciprocating machine in which (a) has in each supply forair of combustion a further flow-restricting element, across whichduring operation, a pressure differential prevails which is proportionalto the rate of the flow of air of combustion through the relevantsupply, and (b) a further selecting device for the automatic selectionof the maximum pressure of air of combustion from several inletpressures of air of combustion. The outlet sides of the furtherflow-restricting elements communicate with the same number of inlets ofthe further selecting device, the pressure difierential between thepressure of air of combustion on the inlet side of the further flowrestricting elements and the selected maximum pressure of air ofcombustion influences the control member in the opposite sense to thefuel pressure differential.

Since the further selecting device selects the maximum pressure of thepressures of air of combustion prevailing on the outlet sides of thefurther flow restricting elements, said device follows the smallest flowof air of combustion of all the flows flowing through the varioussupplies of air of combustion.

By suitable adjustment of the control system, it is achieved that saidminimum flow of air of combustion always is sufficiently large to ensurecomplete combustion of the maximum fuel flow from the various fuel flowspreceding to the various burner devices.

In the present case again, only one general control system is necessaryinstead of an associated control separately for each cylinder and burnerdevice, respectively. As further flow restricting elements are to beconsidered again the above-mentioned simple measuring plates but ifdesirable, for example, Pitot tubes or Venturis may also be used. Justas the selecting device, the further selection device may also be of asimple and compact structure.

In a favourable embodiment of the multicylinder thermodynamicreciprocating machine according to the invention, the selecting deviceand the further selecting device, respectively, consist of a number ofcheck valves which pass only in one direction and which are arrangedbeside each other with their directions of passing oriented mutually inthe same direction. The valves communicate with their one side inletsfor an equal number of inlet pressures and communicating with theirother side with a common outlet, the inlet and outlet sides of eachvalve being always in open communication with each other by means of aleak restriction.

A further favorable embodiment of the multicylinder thermodynamicreciprocating machine according to the invention is characterized inthat each check valve comprises a valve body, a foil element which cancooperate in a sealing manner with a seat, said foil element comprisingan aperture as a leak restriction.

In order that the invention may readily be carried into effect, a fewembodiments thereof will now be de scribed in greater detail, by way ofexample, with reference to the accompanying drawings which arediagrammatic and not drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS machine inwhich, in addition to theselecting device for the maximum fuel flow, a further selecting deviceis present; the minimum flow of air of combustion is selected from thefour flows of air of combustion flowing to the four burner devices, theminimum flow of air of 5 combustion is compared with the maximum fuelflow and the overall flow of air of combustion being derived therefrom.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1,reference numeral 1 denotes a 4-cylinder thermodynamic reciprocatingmachine having with each cylinder a heater (not shown) and an associatedburner device 2. A fuel supply 3 and a supply 4 for air of combustioncommunicate with each burner device 2. All the fuel supplies 3 communicate with a common fuel supply duct 5 in which a fuel pump 6 isincorporated with which fuel can be supplied to the fuel supplies 3 froma fuel container 7.

Communicating with the common fuel supply duct 5 is a fuel return duct 8in which a pressure control valve 9 is incorporated. During operationthis valve maintains a constant pressure in the common fuel supply duct5 and hence also on the inlet sides of the fuel supplies 3 communicatingtherewith.

All the supplies 4 for air of combustion communicate with a commonsupply duct 10 for air of combustion.

- Each of the heaters which are not shown in detail comprise athermocouple 11 as a temperature sensitive element with which duringoperation the heater temperature is determined. The electric signalssupplied by the termocouple 11 are individually amplified in a centralamplifier unit 12. Each amplifiedsignal determines the position of anelectromagnetic valve 13 as a fuel control mechanism in the fuel supply3 of the burner device 2 associated with the relevant heater.

Taken in the direction of flow before the electromagnetic valve 13, aflow restricting element 14 is present in each fuel supply 3 acrosswhich during operation a pressure differential prevails which isproportional to the rate of the fuel flow through the relevant supply.The flow restricting elements may be of a simple construction, forexample, as the known thin plates comprising an aperture. If desirable,Pitot tubes or Venturis or the like may also be used.

A selecting device 15 is present which has four inlets on which the fouroutlet sides of the flow restricting elements l4 communicate throughducts 16. The selecting device 15 selects the minimum pressure of thefour pressures prevailing on the outlet sides of the flow restrictingelements 14; this minimum pressure is supplied to a pressure differencesensor 17 as a control member. The constant pressure which prevails onthe inlet side of the flow restricting elements 14 is also supplieddirectly to the pressure difference sensor 17 via a duct l8.

In the pressure difference sensor 17, the difference in the twopressures supplied to it is converted into an electric signal which isamplified in an amplifier 19. The position of an electromagnetic valve20 as a control mechanism for air of combustion in the common supplyduct id for air of combustion is controlled with the amplified signal.Pressure difference sensor 17 may be constructed, for example, as shownin FIG. 6 to be described hereinafter.

The operation of the air-fuel control system is as follows. Duringoperation, the fuel. pump 6 pumps fuel from the fuel container 7 to thefuel supplies 3, while in a manner not shovim, air of combustion issupplied to the common supply duct It) for air of combustion, which airis distributed between the four supplies 4 for air of combustion. As aresult of the control action of the pressure control valve 9, a constantpressure prevails on the inlet sides of the flow restricting elements14. So a constant pressure signal is supplied to pressure differencesensor l7 via duct 1%.

The fuel flow flowing to a burner device 2 through the relevant fuelsupply 3 depends upon the temperature of the associated heater. When thetemperature of the heater decreases, for example, because more power isconsumed from the relevant cylinder, the relevant thermocouple itensures that the electromagnetic valve 13 in question is opened furtherand more fuel can flow to the burner device. Conversely, when thetemperature of the heater increases, the thermocouple ll. ensures thatthe valve 13 is further closed and less fuel is passed to the burnerdevice.

Since the pressure on the inlet. sides of the flow restricting elementsM is constant and since the pressure drop across such an elementincreases when the fuel flow through said element increases, this meansthat the lowest of the pressures prevailing on the outlet sides of theflow restricting elements 14 prevails in that fuel supply 3 throughwhich the greatest fuel flow passes.

As already described, the selecting device l5 now selects said lowestfuel pressure from the four pressure signals supplied to it via ducts17. The pressure differential which influences the pressure differencesensor 17 and is constituted by the difi'erence between the constantpressure on the inlet side of the flow restricting elements M and theminimum pressure selected from the four pressures prevailing on theoutlet sides of the said elements, therefore represents the maximum fuelflow of the various flows flowing through the four fuel supplies 3 tothe four burner devices 2.

On the basis of this pressure differential representing the maximum fuelflow, the overall quantity of air of combustion supplied collectively tothe four burner devices is controlled in that the electrical signalderived from the pressure differential, after amplification in theamplifier 19, controls the position of the electromagnetic valve 20.

When the air-fuel control system is adjusted so that the overallquantity of air of combustion supplied per unit of time is at leastequal to four times the quantity of air of combustion necessary toensure complete combustion of the maximum fuel flow, then the completecombustion of the other three fuel flows which as a matter of fact aresmaller than the selected fuel flow, is practically always ensured.

The whole control system of the present 4-cylinder thermodynamicreciprocating machine is very simple in construction and consists of avery small number of components. In what manner the selecting device 115may be constructed will be described in greater detail with reference toFIG. 5.

In practice, air of combustion is usually supplied to the common supplyduct for air of combustion by means of a fan which is rigidly coupled toa shaft of the machine. Dependent upon the number of revolutions of theengine, the efficiency of the fan is then varied. In the case in whichthe fan is coupled to a shaft of a multicylinder thermodynamicreciprocating machine as that shown in FIG. 1, it should be ensured thatin the case of a variable supply of supply duct for air of combustion,the quantity of air of combustion passed to the supplies 4 for air ofcombustion remains matched to the maximum fuel flow selected from thevarious flows through the fuel supplies 3.

A solution for this is shown in FIG. 2. The air-fuel control systemshown in FIG. 2 in general is equal to that shown in FIG. 1, so that thesame reference numerals are used for corresponding components. In thepresent case, a fan 21 is present with which air combustion is suppliedto the common supply duct 6 for air of combustion. The shaft of fan 21,is coupled to a shaft of. the machine which is not shown in the drawing.An element 22 restricting the flow of air of combustion is present inthe common supply duct 10 for air of combustion and supplies a pressuredifferential which is proportional to the flow of air of combustionthrough said duct, which pressure differential is supplied to a pressuredifferences sensor 23. The pressure differential supplied to said sensoris converted in it into an electric signal which influences an electriccomparison element 24 as a control member in a sense opposite to theelectric signal originating from the pressure difference sensor 17.

Comparison element 24 controls the position of the electromagnetic valve20' on the basis of the difference between the two electric signalssupplied to said element. When the number of revolutions of the fanincreases and hence the quantity of air which passes the valve 20'increases, the value of the pressure differential signal supplied byelement 22 restricting the flow of air of combustion also increases andhence the electric signal derived therefrom. Comparison element 24ensures that the value 20 is closed to such an extent that the originalquantity of air of combustion again passes valve 20. When the number ofrevolution of the fan decreases and hence also the quantity of air whichpasses the valve 20, the element 24 ensures that the valve 20' is openedto a greater extent so that the original air flow is maintained.

Element 22 restricting the flow of air of combustion may be, forexample, a Pitot tube or a Venturi, may be constructed from one or moreplates provided with apertures, but it may also be constructed as ananemometer. When an electric anemometer is used, the electric signalwhich is derived herefrom and is proportional to the flow of air ofcombustion can be directly supplied to the comparison element 24. Thecharacteristics of said, anemometer and those of the pressure differencesensors 17 should then be matched mutually.

The pressure difference sensors 17' and 23 may be constructed as isshown by way of example in FIG. 6 to be described hereinafter. Acombined unit, such as the one shown in FIG. 7 to be describedhereinafter, may be used for the two pressure sensors together with thecomparison element 24.

In the air-fuel control systems shown in FIGS. l and 2, the overall flowof air of combustion is matched to the maximum fuel flow of the variousflows through the fuel supplies 3 in that the selecting device 15selects the minimum pressure from the various pressures prevailing onthe outlet sides of the flow resisting elements M.

In the case of small fuel flows through the fuel supplies 3, thepressure differentials between the inlet and outlet sides of the flowrestricting elements M are small. Small pressure difference signals areunfavourable for an accurate air-fuel control. In addition it issubstantially impossible in the preferably used measuring plates tocause the flow through the plates to be turbulent and hencetemperature-independent in the case of small fuel flows. All thisresults in an inaccurate air-fuel control in the case of small fuelflows, which results in too low air-fuel ratios with said small fuelflows and consequently in incomplete combustion with dirty exhaustgases.

This problem is no longer present in the airfuel control system shown inFIG. 3 which in general is equal to that shown in FIG. I and in whichtherefore the same reference numerals are used for corresponding components.

The control system shown in FIG. 3 differs from that shown in FIG. 1 inthat in the present case a fuel auxiliary duct 30 is present whichcommunicates at one end with the common fuel supply duct 5" and at itsother end opens into the fuel container 7. A flow restricting element14" the outlet side of which also communicates, via a duct 16', with anextra, fifth, inlet of the selecting device 15 which, as stated selectsthe minimum pressure, is also present in the fuel auxiliary duct 30, asin the fuel supplies 3". Furthermore, a fixed restriction 31 isincorporated in the fuel auxiliary duct 30.

Since during operation a constant pressure prevails on the inlet side ofthe fuel auxiliary duct 30, a constant pressure will prevail also insaid duct as a result of the presence of the fifth restriction 31 on theoutlet side of the flow restricting element 14" and a constant fuel flowwill flow through fuel auxiliary duct 30 to fuel container 7.

Fixed restriction 3i and hence the fuel flow flowing away through thefuel auxiliary duct 30 is now chosen to be so that said fuel flowcorresponds to a certain minimum value of the fuel flows through thevarious fuel supplies 3". Above said certain minimum value, all the fuelflows through the fuel supplies are greater than the constant fuel flowthrough the fuel auxiliary duct 30. In that case the pressures on theoutlet sides of the flow restricting element 1 in the fuel supplies 3"are all lower than the constant pressure on the outlet side of the flowrestricting element M in the fuel auxiliary duct 30. Since the selectingdevice 15 selects the smallest from the five supplied pressure signals,said minimum pressure signal is always one of the four pressure signalsoriginating from the fuel supplies 3". The fuel auxiliary duct 30 isthen passive relative to the airfuel control.

If all the fuel flows through the fuel supplies 3' however, fall belowthe minimum value which corresponds to the constant fuel flow throughthe fuel auxiliary duct 30, then, since the last-mentioned flow then isthe greatest, the pressure on the outlet side of the flow restrictingelement M" in the fuel auxiliary duct 30 is lower than all the pressuresprevailing on the outlet sides of the flow restricting element 14" inthe fuel supplies 3". The selecting device 15" then selects as a minimumpressure the pressure signal originating from the fuel auxiliary duct30. Since this signal is constant owing tothe constant pressure on theoutlet side of the flow restricting element 14" in said duct, a constantsignal is supplied to the pressure difference sensor 17''. This meansthat the overall quantity of air of combustion supplied per unit timethen remains constant since actually the position of the electromagneticvalve 20 no longer varies.

In this manner it is achieved that below a certain minimum value for thefuel flows through the fuel flows through the fuel supplies 3", theoverall flows of air of combustion remains constant irrespective of thevalue which the said fuel flows then assume. If, when the fuel flowsthrough the fuel supplies 3" have values which are equal to the minimumvalue, the overall flow of air of combustion is sufficiently large toobtain complete combustion of the said flows, then said overall air flowis certainly sufficiently large for complete combustion of fuel flowsthrough the fuel supplies below said minimum value. Complete combustionis therefore substantially always ensured while unreliable measurementof small fuel flows through the fuel supplies 3" is no longer necessaryand is avoided. g

In the air-fuel control systems shown in FIGS. 1 to 3, the overall flowof air of combustion is always controlled in accordance with the maximumfuel flow through one of thefuel supplies 3" and the maximum fuel belowflow of the variable flows through the fuel supplies 3", respectively,and the constant flo through the fuel auxiliary duct 30. i

The starting idea is that the overall flow of air of combustion throughthe common supply duct 10" for air of combustion distributes uniformlybetween the four supplies 4" for air of combustion. In circumstances,however, it may occur that this latter is not always the case to asufficient extent. The cause hereof may be the mutually different flowrestriction values of the supplies 4" for air of combustion. It is alsopossible that from the burner devices 2 mutually different counterpressures influence the supplies for air of combustion. The result ofall this may be that too little air of com bustion is supplied to asupply for air of combustion to ensure complete combustion of the fuelsupplied to the corresponding fuel supply as a result of which dirtyexhaust gases of the relevant burner device are obtained.

In the air-fuel control system shown in FIG. 4 this drawback is avoided.For this system, the same reference numerals are used for componentscorresponding to the system shown in FIG. 3. The present system differsfrom that shown in FIG. 3 in the following points. a

for air of combustion.

The four outlet-sides of the further flow restricting elements 40 areconnected to the same number of inlets of further selecting device 41via ducts d2.

The common pressure prevailing on the inlet sides of the further flowrestricting elements 40, namely the pressure in the common supply duct10a for air of combustion as well as the maximum: pressure selected bythe further selecting device 41 from the various pressures prevailing onthe outlet sides of the said elements, are individually supplied to apressure difference sensor 43. The difference between the two pressuresis con-- verted in said sensor into an electric signal which influencesan electric comparison element 44- in a sense opposite to the electricsignal originating from the pressure difference sensor 170. Comparisonelement 44 operates the electromagnetic valve 2% on the basis of thedifference between the electric signals supplied to said element.

Pressure difference sensor 43 has a characteristic which is equal tothat of the pressure difference sensor 17a. Both pressure differencesensors may again be constructed; for example, as shown in FIG. 6 or maybe constructed as one unit together with the comparison element 44, forexample, in the manner shown in FIG. 7.

Since the further selecting device 41 selects the maximum pressure fromthe various pressures prevailing on the outlet sides of the further flowrestricting elements 40, this means that the selected maximum pressureoriginates from that supply 4a for air of combustion through which thesmallest flow of air of combustion passes. As a matter of fact, thepressure drop across the further flow restricting element 40 present insaid supply for air of combustion is smallest there, while the samecommon pressure prevails on the inlet sides of all the further flowrestricting elements 40. The pressure differential supplied to pressuredifference sensor 43 therefore represents the smallest flow of air ofcombustion of all the flows passing through the supplies 4a for air ofcombustion. All this means that the smallest flow of air of combustionselected from all the flows passing through the various supplies 4a forair of combustion is controlled in accordance with the maximum fuel flowselected by means of the selecting device 15a from the variable fuelflows through the various fuel supplies 3a and the constant fuel flowthrough the fuel auxiliary duct 30.

When the control systemis adjusted so that the selected minimum flow ofair of combustion through a supply 4a for air of combustion issufiiciently large to be able to completely burn the selected maximumfuel flow through a fuel supply 3a and through the fuel auxiliary duct30a, respectively, it is automatically ensured in all circumstances thatall the fuel flows through the fuel supplies 3a are completely burnt bythe flows of air of combustion through the corresponding supplies 4a ofair of combustion. The possible cases then are that the minimum flow ofair of combustion or a larger flow of air of combustion is supplied tothe maximum fuel flow or a smaller fuel flow.

The operation of the present control system is further the same as thatshown in FIG. 3 so that further description may be omitted. Of course,the control system in the present case may be constructed, if desirable,without a fuel auxiliary duct 30a. In addition to the further flowrestricting elements 40, only one further selecting device 41 isnecessary which can be of a compact and simple construction again. Aseparate control for each burner device separately is thus prevented sothat the overall control system remains constructed from a comparativelysmall number of cheap components.

As in the control system shown in FIG. 2, air of combustion can besupplied to the common supply duct a for air of combustion in this casealso by means of a fan which is coupled directly to a shaft of themachine and the efficiency of which thus varies when the number ofrevolutions varies. It is to be noted that in that case, and not only inthat case, it is not always necessary to maintain a constant pressure inthe common supply duct 10a for air of combustion and hence at the inletsides of the further flow restricting elements 40. For example, when thenumber of revolutions of the machine and thus the number of revolutionsof the fan increases, as a result of which the overall supplied quantityof air of combustion and the pressure on the inlet sides of the furtherflow restricting elements 40 also increases, then the pressure on theoutlet sides of the said elements increases to the same extent. Thepressure differential supplied to the pressure difference sensor 42 thenremains unvaried so that the only practical result is that a greaterexcess of air of combustion is supplied to the burner devices 2a. Thisneed by no means be disturbing but may in certain circumstances even beadvantageous. It should be ensured only in the first instance that theselected minimum flow of air of combustion is chosen to be sufficientlylarge relative to the selected maximum fuel flow.

In the control system shown in FIGS. 1 to 4, pressure differentials arealways converted into electric signals. It is of course also possible tosupply the fuel pressure differences directly to the control mechanismin the common supply duct for air of combustion, which control mechanismmay then be constructed as a hydraulically controlled valve. It is alsopossible to compare the fuel pressure differential and the pressuredifferential of air of combustion directly with each other, for example,in a hydraulic comparison element and to cause the difference betweenthe two pressure difference signals to operate the control mechanism inthe common supply duct for air of combustion constructed as ahydraulically controlled valve, for example, in a manner analogous tothat described in British Pat. specification No. 895,869 for the directhydraulic comparison of the flows of fuel and air of combustion to theburner device of a monocylinder thermodynamic reciprocating machine. Incontrast with what is the case in said known monocylinder thermodynamicreciprocating machine, only one comparison element is necessary in thepresent multicylinder thermodynamic reciprocating machine, irrespectiveof the number of cylinders.

Furthermore, a four-cylinder thermodynamic reciprocating-machine isalways shown in the control systems shown .in FIGS. 1 to 4. Of course,the systems shown in the said Figures may equally readily be applied tomulticylinder thermodynamic reciprocating machines having a differentnumber of cylinders while maintaining the advantages. Extension to, forexample, 8-cylinder thermodynamic reciprocating machines only means acomparatively small increase of the overall number of requiredcomponents since only one selecting device and further selecting device,respectively, will always be sufficient.

How such a selecting device and further selecting device, respectively,may be constructed is shown in FIG. 5a, 5b and 50. FIGS. 5a and 5b showselecting devices for the automatic selection of the minimum pressurefrom four inlet pressure signals, while FIG. 5c shows a selecting devicewhich automatically selects the maximum pressure from four inletpressures. Reference numeral in FIG. 5a denotes a housing in which fourcheck valves 51 passing in the same direction are arranged beside eachother. Each check valve 51 comprises a spherical valve body 52 which cancooperate in a sealing manner with a valve seating 53. Ducts 54 passedthrough the wall of the housing 50 constitute the inlet sides of thevalves. Outlet sides of the valves constituted by ducts 55 communicatewith a common outlet 56 which is passed to the outside through the wallof the housing. The inlet side 54 and the outlet side 55 each valve 51are in open communication with each other via a duct 57 in which a leakrestriction is incorporated. The operation of this selection device isas follows.

When the pressures in'the ducts 54, so on the inlet sides of the valves51, are all higher than in the ducts 55 and the common outlet 56,respectively, the valve bodies 52 are forced against the valve seatings53 in a sealing manner. When, however, in only one of the ducts 54 apressure occurs which is lower than the pressure in the common outlet56, the check valve 51 which communicates with the said duct 54immediately releases the communication between said duct 54 and therelevant duct 55 in that the valve body 52 in question moves away fromits seating 53 (upwards) under the influence of the occurring pressuredifferential. The lower pressure which prevails in the said duct 5 thenadjusts in the common outlet 56, while the remaining check valves 51remain closed. In this manner the minimum pressure selected from thevarious inlet pressures prevails in the common outlet 56.

The leak restrictions 58 ensure that when the pressure in the ducts 54increases, the pressure in the common outlet 56 can slowly follow saidincreases in pressure so that in the stationary condition the pressurein the outlet 56 does not differ essentially from the lowest ones of theinlet pressures prevailing in the ducts 54. The extent to which leakagecan occur and hence the choice of the leak restriction depends upon theuse. For the selection device 15 of the control system shown in FIG. I,it is in this connection for example the specific variation in volume ofthe chamber of pressure difference sensor 17 communicating with thecommon outlet that plays a part, in which the permissible deviationinthe pressure sensed by the pressure sensor from the actual minimumpressure should be observed.

For the selecting device shown in FIG. 5b the same reference numeralsare used as for that shown in F IG. 5a. In this case, the valve bodies52 consist of foil elements which can cooperate with the seatings 53 ina sealing manner. Leak restrictions 58 consist of apertures in the foilelements provided in such manner that they cannot be sealed by theseatings 53. The operation of this selection device is the same as thatshown in FIG. 5a so that it need not be described here.

The selecting device shown in FIG. 50 selects the maximum pressure fromseveral (four) inlet pressures. For components corresponding to those ofFIG. 5a the same reference numerals are used. The only difference fromthe selecting device shown in FIG. 5a is that in the present case thecheck valves all pass in the opposite I direction from the check valvesshown in FIG. a.

When all the pressures in ducts 54 are lower than in ducts 52 and commonoutlet 56, respectively, the valve bodies 52 remain forced against theseatings 53 in a sealing manner. If, however, in only one of the ducts54 a pressure occurs which is higher than the pressure in y the commonoutlet 56, the check valve 51 which com- 56 does not differ essentiallyfrom the highest ones of I the inlet pressures prevailing in the ducts54.

The selecting devices described present the advantages of simplicity inconstruction, compactness and reliable operation. Extension to, forexample, eight or nine inlets desirable for, for example, an 8-cylinderthermodynamic reciprocating machine, is possible in a simple mannerwhile maintaining the advantages.

In addition to the embodiments shown, all kinds of other embodiments ofthe selecting devices are of course possible. Reference numeral 60 inFIG. 6 denotes a housing in which a diaphragm 61 is accommodated whichis secured to the housing and which separates a chamber 62 from achamber 63. Chamber 62 is admissible via an inlet 64, chamber 63 via aninlet 65.

The diaphragm 61 supports a magnetic element 66 end a magnetic element84 which faces a soft iron core 85 with induction coil 86 arrangedwithin the chamber 72 and to which electric conductors 87 are connectedwhich are passed to the outside through the wall of the housing 70. Whenthe rod 78 again moves in the direction of the assembly core 85,induction coil 86, an electric signal is induced in this case also inthe coil 86 the value of which is proportional to the distance overwhich the rod 78 has moved.

By supplying to the chambers 72 and 76 the pressure differential Ap,which represents a flow of air of combustion and to the chambers 73 and75 the pressure differential A p which represents a fuel flow, aequilibrium condition is obtained in which the rod 78 assumes a givenposition with a corresponding electric signal from the induction coil86. The forces on the rod as a result of the pressure differentialprevailing across the diaphragms 7d and 77 then make equilibrium withthe forces on the said rod as a result of the tension forces in thediaphragm.

When pressure differential A p varies, the equilibrium of forces isdisturbed and the rod 78 assumes a new position in which a newequilibrium of forces is achieved. The induction coil 86 then supplies anew electric signal corresponding to the new position. i

The control mechanism 20 in the common supply duct 10 for air ofcombustion of FIGS. 2 and 4, respec which faces a soft iron core 67 withinduction coil 68 arranged inside the chamber 63. Electric conductors 69are connected to the induction coil and are passed to the outsidethrough the wall of the housing 60.

When the magnetic element 66 moves in the direction of the core 67, anelectric signal the value of which is proportional to the distance overwhich the magnetic element 66 moves, is produced in the induction coil68.

The two different pressures can be supplied to the inlets 64 and 65 andare provided, for example, by the selecting device 15 and the commonfuel supply duct 15 of FIG. 1, in which the present device serves as apressure difference sensor 17.

When the pressure difi'erential varies, the electric signal of theinduction coil 68 varies proportionally thereto in that the pressuredifferential across the diaphragm 61 varies and said diaphragm movestowards or away from the core 67.

FIG. 7 shows a housing 70 having a partition 71 which divides the spacewithin the housing into two sub-spaces. One sub-space consists of twochambers 72 and 73 separated from each other by a diaphragm 74, whilethe other sub-space consists of two chambers 75 and 76 separated fromeach other by a diaphragm 77.

Diaphragms 74 and 77 are connected at one end to the housing 70 and atthe other end to a common rod 78 which can reciprocate axially and ispassed through the partition 71 via an aperture 79.

Chambers 72, 73, 75 and 76 each comprise inlets 80, 81, 82 and 83,respectively. The rod 78 supports at one tively, can be controlleddirectly with the signal supplied by the induction coil 86.

The same medium is present in the chambers 73 and 75. Any medium leakfrom the higher to the lower pressure chamber consequently provides nocomplications while the pressure differential between the chambers ishardly influenced by small leakage.

What is claimed is:

l. A multicylinder thermodynamic reciprocating ma chine in which eachcylinder comprises a heater to which thermal energy from an associatedburner device can be supplied, in which a fuel supply and a supply forair of combustion communicate with each burner device, in which the endsof the fuel supplies remote from the associated burner devicecommunicate with a common fuel supply duct in which a pressure controldevice maintains a constant fuel pressure during operation and the endsof the supplies for air of combustion remote from the associated burnerdevice communicate with a common supply duct for air of combustion, eachheater comprising a temperature-sensitive element which actuates a fuelcontrol mechanism in the fuel supply of the associated burner device forcontrolling the fuel flow to the said device, each fuel supply, taken inthe direction of flow, comprising before the fuel control mechanism aflow restricting element across which during operation a pressuredifferential prevails which is proportional to the rate of fuel flowthrough the relevant supply, the quantity of air of combustion to besupplied being controlled in accordance with the supplied quantity offuel, characterized in that a selecting device is present for theautomatic selection of the maximum or minimum fuel pressure from severalfuel inlet pressure and the outlet sides of the flow restrictingelements communicate with an equal number of inlets of the selectingdevice, the pressure differential between the constant fuel pressure inthe common fuel supply duct and the selected maximum or minimum fuelpressure influencing a control member which actuates a control mechanismfor air of combustion in the common supply duct for air of combustion.

2. Amulticylinder thermodynamic reciprocating machine as claimed inclaim 1, characterized in that a fuel auxiliary duct is present whichcommunicates with one end with the common fuel supply duct, in whichauxiliary duct a flow restricting element is also incorporated von theinlet side of which the constant fuel pressure prevails and the outletside of which also communicates with an inlet of the selecting device,which selecting device selects the minimum fuel pressure, the fuelauxiliary duct furthermore comprising, taken in the direction of flowafter the flow restricting element, a fixed restriction which is chosento be so that below a given minimum fuel flow to the fuel supplies thelikewise constant pressure prevailing on the outlet side of the flowrestricting element in the fuel auxiliary duct is lower than thepressures which prevail in that case on the outlet sides of the flowrestricting elements in the fuel supplies.

3. A multicylinder thermodynamic reciprocating machine as claimed inclaim 1 in which a further flow restricting element is present in eachsupply for air of combustion and across which during operation apressure differential prevails which is proportional to the rate of theflow of air of combustion through the relevant supply, characterized inthat a further selecting device is present for the automatic selectionof the maximum pressure of air of combustion from several inletpressures 'of air of combustion and the outlet sides of the further flowrestricting elements communicate with an equal same number of inlets ofthe further selecting device, the pressure differential between thepressure of air of combustion on the inlet side of the further flowrestricting elements and the selected maximum pressure of air ofcombustion influencing the control member in the opposite sense to thefuel pressure differential.

4. A multicylinder thermodynamic reciprocating machine as claimed inclaim 1, characterized in that the selecting device and furtherselecting device, respectively, consist of a number of check valveswhich pass only in one direction and which are arranged beside eachother with their directions of passage mutually in the same direction,the valves constituting with their one side inlets for an equal numberof inlet pressure and communicating with their other sides with a commonoutlet, the inlet and outlet sides of each valve being always in opencommunication with each other by means of a leak restriction.

5. A multicylinder thermodynamic reciprocating machine as claimed inclaim 4, characterized in that each check valve comprises as a valvebody a foil element which can cooperate in a sealing manner with aseating, said foil element comprising an aperture as a leak restriction.

6. In a thermodynamic engine, operable with sources of fuel and air, theengine having a plurality of cylinders, each of which has at least onereciprocating piston therein, a heater for providing thermal energy toeach cylinder, a burner for supplying thermal energy to each heater fueland air supply ducts respectively feeding fuel and air to said burnersfrom said fuel and air sources, the improvement in combination therewithof means for controlling the amounts of fuel and air supplied to theburners comprising: a common fuel duct feeding the fuel supply ductsfrom the fuel source, a

common air duct feeding the air supply ducts from the air source, firstmeans associated with the fuel supply and common fuel duct formaintaining substantially constant pressure on said fuel, second meansfor measuring the temperature in each heater and providing acorresponding signal, a flow restrictor in each fuel supply ductincluding third means for measuring the pressure in the downstream sideof the restrictor, said downstream pressure inversely corresponding tothe rate of flow through the restrictor, a first flow control valve ineach fuel supply duct intermediate said restrictor and the burner, asecond flow control valve in the common air duct, fourth means to whichis communicated the pressures from said third means, and which selectsthe lowest pressure, fifth means receiving said lowest pressure from thefourth means, further duct means communicating the constant pressurefrom the common fuel duct to said fifth means which compares anddetermines the difference between said lowest pressure and said constantpressure and provides a resultant output signal, sixth meanscommunicating said output signal to said second flow control valve foradjusting said valve, seventh means receiving the temperature signalsfrom said second means associated with each heater and providing aninversely corresponding control signal to the first flow control valveassociated with that heater, whereby lower temperature in a heatercauses the seventh means to open the first flow control valve in thesupply fuel duct, which greater flow produces a greater pressure dropacross the flow restrictor and a lower pressure reading by the thirdmeans, which low pressure is selected by the fourth means and comparedwith the constant pressure by the fifth means which causes the secondflow control valve in the common air duct to be opened correspondingly.

7. Apparatus according to claim 6, further comprising an air-flowrestrictor in said common air duct through which the air-flow has apressure differential, eighth means communicating with said air-flowrestrictor and sensing and measuring said pressure differential andproviding a corresponding signal, a comparison means receiving saidsignal from the eighth means and said signal from said fifth meanscorresponding to the fuel flow pressure differential, the comparisonmeans having a resultant output controlling the second flow controlvalve in said common air duct, whereby (i) a greater air-flow throughthe common air duct produces a greater pressure differential throughsaid airflow restrictor, said eighth means providing a signalcorresponding to said pressure differential for adjusting said secondflow control valve to reduce the air-flow therethrough, and (ii) asmaller air-flow through said common air duct produces a reverse effect.

8. Apparatus according to claim 6 further comprising an auxiliary fuelduct including a flow restrictor therein communicating said common fuelsupply duct with said fuel source, and further duct means communicatingsaid common fuel supply duct with said fourth means, whereby air-flowwill be constant regardless of fuel flow below predetermined minimumvalve of said fuel flow.

9. Apparatus according to claim 8 further comprising an air-flowrestrictor in each air supply duct, means for measuring the pressure inthe downstream side of each of said air-flow restrictors, said downsreampressure imversely corresponding to the rate of flow through the airestrictor, means for selecting the maximum pressure, means for comparingsaid maximum pressure with the pressure in said common supply duct andproviding a corresponding signal to said comparison means and inopposite sense to the signal of said fifth means that determines fuelpressure differential, said comparison means controlling said flowcontrol valve in the common air duct.

mg v UNITED. STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,822,550 Dated ly 1974 Inventor) KLAUS BRANDENBERG ET AL It iscertified that error appears in theabove-identified patent and that saidLetters Patent are hereby corrected as shown below:

fl IN THE TITLE PAGE:

In the "FOREIGN APPLICATION PRIORITY DATA" the foreign applicationnumber should be --7lO36l0--.

Col. 1, line 26, change "A" to --a- Col. 4, line 47, after "comprises"insert -as-- Col. 7, line 22, delete "6"" and insert -lO'- Claim 5, line5, "imversely" should be --inversely-;

line 6, "estrictor" should be -restrictor'- Signed and sealed this 3rdday of June 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officerand Trademarks

1. A multicylinder thermodynamic reciprocating machine in which eachcylinder comprises a heater to which thermal energy from an associatedburner device can be supplied, in which a fuel supply and a supply forair of combustion communicate with each burner device, in which the endsof the fuel supplies remote from the associated burner devicecommunicate with a common fuel supply duct in which a pressure controldevice maintains a constant fuel pressure during operation and the endsof the supplies for air of combustion remote from the associated burnerdevice communicate with a common supply duct for air of combustion, eachheater comprising a temperature-sensitive element which actuates a fuelcontrol mechanism in the fuel supply of the associated burner device forcontrolling the fuel flow to the said device, each fuel supply, taken inthe direction of flow, comprising before the fuel control mechanism aflow restricting element across which during operation a pressuredifferential prevails which is proportional to the rate of fuel flowthrough the relevant supply, the quantity of air of combustion to besupplied being controlled in accordance with the supplied quantity offuel, characterized in that a selecting device is present for theautomatic selection of the maximum or minimum fuel pressure from severalfuel inlet pressure and the outlet sides of the flow restrictingelements communicate with an equal number of inlets of the selectingdevice, the pressure differential between the constant fuel pressure inthe common fuel supply duct and the selected maximum or minimum fuelpressure influencing a control member which actuates a control mechanismfor air of combustion in the common supply duct for air of combustion.2. A multicylinder thermodynamic reciprocating machine as claimed inclaim 1, characterized in that a fuel auxiliary duct is present whichcommunicates with one end with the common fuel supply duct, in whichauxiliary duct a flow restricting element is also incorporated on theinlet side of which the constant fuel pressure prevails and the outletside of which also communicates with an inlet of the selecting device,which selecting device selects the minimum fuel pressure, the fuelauxiliary duct furthermore comprising, taken in the direction of flowafter the flow restricting element, a fixed restriction which is chosento be so that below a given minimum fuel flow to the fuel supplies thelikewise constant pressure prevailing on the outlet side of the flowrestricting element in the fuel auxiliary duct is lower than thepressures which prevail in that case on the outlet sides of the flowrestricting elements in the fuel supplies.
 3. A multicylinderthermodynamic reciprocating machine as claimed in claim 1 in which afurther flow restricting element is present in each supply for air ofcombustion and across which during operation a pressure differentialprevails which is proportional to the rate of the flow of air ofcombustion through the relevant supply, characterized in that a furtherselecting device is present for the automatic selection of the maximumpressure of air of combustion from several inlet pressUres of air ofcombustion and the outlet sides of the further flow restricting elementscommunicate with an equal same number of inlets of the further selectingdevice, the pressure differential between the pressure of air ofcombustion on the inlet side of the further flow restricting elementsand the selected maximum pressure of air of combustion influencing thecontrol member in the opposite sense to the fuel pressure differential.4. A multicylinder thermodynamic reciprocating machine as claimed inclaim 1, characterized in that the selecting device and furtherselecting device, respectively, consist of a number of check valveswhich pass only in one direction and which are arranged beside eachother with their directions of passage mutually in the same direction,the valves constituting with their one side inlets for an equal numberof inlet pressure and communicating with their other sides with a commonoutlet, the inlet and outlet sides of each valve being always in opencommunication with each other by means of a leak restriction.
 5. Amulticylinder thermodynamic reciprocating machine as claimed in claim 4,characterized in that each check valve comprises as a valve body a foilelement which can cooperate in a sealing manner with a seating, saidfoil element comprising an aperture as a leak restriction.
 6. In athermodynamic engine, operable with sources of fuel and air, the enginehaving a plurality of cylinders, each of which has at least onereciprocating piston therein, a heater for providing thermal energy toeach cylinder, a burner for supplying thermal energy to each heater,fuel and air supply ducts respectively feeding fuel and air to saidburners from said fuel and air sources, the improvement in combinationtherewith of means for controlling the amounts of fuel and air suppliedto the burners comprising: a common fuel duct feeding the fuel supplyducts from the fuel source, a common air duct feeding the air supplyducts from the air source, first means associated with the fuel supplyand common fuel duct for maintaining substantially constant pressure onsaid fuel, second means for measuring the temperature in each heater andproviding a corresponding signal, a flow restrictor in each fuel supplyduct including third means for measuring the pressure in the downstreamside of the restrictor, said downstream pressure inversely correspondingto the rate of flow through the restrictor, a first flow control valvein each fuel supply duct intermediate said restrictor and the burner, asecond flow control valve in the common air duct, fourth means to whichis communicated the pressures from said third means, and which selectsthe lowest pressure, fifth means receiving said lowest pressure from thefourth means, further duct means communicating the constant pressurefrom the common fuel duct to said fifth means which compares anddetermines the difference between said lowest pressure and said constantpressure and provides a resultant output signal, sixth meanscommunicating said output signal to said second flow control valve foradjusting said valve, seventh means receiving the temperature signalsfrom said second means associated with each heater and providing aninversely corresponding control signal to the first flow control valveassociated with that heater, whereby lower temperature in a heatercauses the seventh means to open the first flow control valve in thesupply fuel duct, which greater flow produces a greater pressure dropacross the flow restrictor and a lower pressure reading by the thirdmeans, which low pressure is selected by the fourth means and comparedwith the constant pressure by the fifth means which causes the secondflow control valve in the common air duct to be opened correspondingly.7. Apparatus according to claim 6, further comprising an air-flowrestrictor in said common air duct through which the air-flow has apressure differential, eighth means communicating with said air-flowrestrictor and sensing and measuRing said pressure differential andproviding a corresponding signal, a comparison means receiving saidsignal from the eighth means and said signal from said fifth meanscorresponding to the fuel flow pressure differential, the comparisonmeans having a resultant output controlling the second flow controlvalve in said common air duct, whereby (i) a greater air-flow throughthe common air duct produces a greater pressure differential throughsaid airflow restrictor, said eighth means providing a signalcorresponding to said pressure differential for adjusting said secondflow control valve to reduce the air-flow therethrough, and (ii) asmaller air-flow through said common air duct produces a reverse effect.8. Apparatus according to claim 6 further comprising an auxiliary fuelduct including a flow restrictor therein communicating said common fuelsupply duct with said fuel source, and further duct means communicatingsaid common fuel supply duct with said fourth means, whereby air-flowwill be constant regardless of fuel flow below predetermined minimumvalve of said fuel flow.
 9. Apparatus according to claim 8 furthercomprising an air-flow restrictor in each air supply duct, means formeasuring the pressure in the downstream side of each of said air-flowrestrictors, said downsream pressure imversely corresponding to the rateof flow through the air estrictor, means for selecting the maximumpressure, means for comparing said maximum pressure with the pressure insaid common supply duct and providing a corresponding signal to saidcomparison means and in opposite sense to the signal of said fifth meansthat determines fuel pressure differential, said comparison meanscontrolling said flow control valve in the common air duct. 10.Apparatus according to claim 6 wherein said sixth means furthercomprises means for amplifying the signal from the fifth means. 11.Apparatus according to claim 6 wherein said engine has an output shaft,said air source is driven by said shaft.